Describe valid project name on error

PR #261 changed `docker compose` behavior to require normalized project
names as input, instead of normalizing project names automatically. This
landed in compose-spec/compose-go v1.2.5 and docker/compose v2.5.1.

However, the previous error message gave no indication why a name isn't
valid.  This is surprising for users whose previously working project
names no longer work with newer versions of `docker compose`.

As of compose-spec/compose-spec#314, the spec now contains the text:

> Project name MUST contain only lowercase letters, decimal digits,
> dashes, and underscores, and MUST begin with a lowercase letter or
> decimal digit.

This updated error message provides a concise version of this
requirement based on regular expression character range notation.

See also:

- compose-spec/compose-spec#311
- docker/compose#9741

Signed-off-by: Mike Bland <mbland@acm.org>

cli/options.go

@@ -65,7 +65,8 @@ func NewProjectOptions(configs []string, opts ...ProjectOptionsFn) (*ProjectOpti
 func WithName(name string) ProjectOptionsFn {
 	return func(o *ProjectOptions) error {
 		if name != loader.NormalizeProjectName(name) {
-			return fmt.Errorf("%q is not a valid project name", name)
+			return fmt.Errorf("%q is not a valid project name: it must contain " +
+				"only characters from [a-z0-9_-] and start with [a-z0-9]", name)
 		}
 		o.Name = name
 		return nil

cli/options_test.go

@@ -54,7 +54,7 @@ func TestProjectName(t *testing.T) {
 
 	t.Run("by name start with invalid char '-'", func(t *testing.T) {
 		_, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithName("-my_project"))
-		assert.Error(t, err, `"-my_project" is not a valid project name`)
+		assert.ErrorContains(t, err, `"-my_project" is not a valid project name`)
 
 		opts, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithEnv([]string{
 			fmt.Sprintf("%s=%s", consts.ComposeProjectName, "-my_project"),
@@ -67,7 +67,7 @@ func TestProjectName(t *testing.T) {
 
 	t.Run("by name start with invalid char '_'", func(t *testing.T) {
 		_, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithName("_my_project"))
-		assert.Error(t, err, `"_my_project" is not a valid project name`)
+		assert.ErrorContains(t, err, `"_my_project" is not a valid project name`)
 
 		opts, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithEnv([]string{
 			fmt.Sprintf("%s=%s", consts.ComposeProjectName, "_my_project"),
@@ -80,7 +80,7 @@ func TestProjectName(t *testing.T) {
 
 	t.Run("by name contains dots", func(t *testing.T) {
 		_, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithName("www.my.project"))
-		assert.Error(t, err, `"www.my.project" is not a valid project name`)
+		assert.ErrorContains(t, err, `"www.my.project" is not a valid project name`)
 
 		opts, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithEnv([]string{
 			fmt.Sprintf("%s=%s", consts.ComposeProjectName, "www.my.project"),
@@ -93,7 +93,7 @@ func TestProjectName(t *testing.T) {
 
 	t.Run("by name uppercase", func(t *testing.T) {
 		_, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithName("MY_PROJECT"))
-		assert.Error(t, err, `"MY_PROJECT" is not a valid project name`)
+		assert.ErrorContains(t, err, `"MY_PROJECT" is not a valid project name`)
 
 		opts, err := NewProjectOptions([]string{"testdata/simple/compose.yaml"}, WithEnv([]string{
 			fmt.Sprintf("%s=%s", consts.ComposeProjectName, "_my_project"),